In recent years, there is a demand for an instant access by a high-speed search as well as a long time recording by a high recording density in the field of digital magnetic recording and reproducing apparatuses using a magnetic tape medium (for example, a digital VCR). When performing a high-speed search, it is necessary to control the rotational speed of a rotating drum according to the feed speed of the magnetic tape so that the track direction component of the relative speed of the magnetic tape to a rotating magnetic head becomes substantially the same as that in normal reproduction (i.e., the relative speed becomes uniform).
For example, when performing a high-speed search in an FF direction (a direction in which the magnetic tape is fast forwarded), the relative speed is made uniform by rotating the rotating drum at a higher speed than the drum rotational speed in normal reproduction. On the other hand, when performing a high-speed search in a REW direction (a direction in which the magnetic tape is rewound), the relative speed is made uniform by rotating the rotating drum at a lower speed than the drum rotational speed in normal reproduction. The rotational speed of the rotating drum will be explained in detail below by presenting a specific example using a HD specification and recommendation values determined by the HD Digital VCR Council. When performing a high-speed search at a speed 200 times higher than that in normal reproduction which is executed at a drum rotational speed of 9,000 min.sup.-1 ! and a magnetic tape feed speed of 37.6 mm/sec.!, the relative speed is made uniform by controlling the drum rotational speed so that the rotational speed becomes about 15,500 min.sup.-1 ! in the FF direction search and about 2,400 min.sup.-1 ! in the REW direction search.
In general, the drum is provided with a rotational speed detector (hereinafter referred to as the "FG"), and a rotational position detector (hereinafter referred to as the "PG"). The FG generates a plurality of pulses per rotation of a rotor (rotating section) so as to detect the number of rotations of the drum. The PG generates one pulse per rotation of the rotor so as to detect the absolute positional relationship between the rotating drum and the magnetic head.
The control of the rotational speed of the rotating drum is usually accomplished by a phase loop using a PG pulse produced by converting the PG detection pulse into a binary form, and a velocity loop using FG pulses produced by converting the FG detection signals into a binary form. Therefore, when rotating the drum while accurately controlling its rotational speed over a broad speed range from a low speed of 2,400 min.sup.-1 ! to a high speed of 15,500 min.sup.-1 ! like the above-mentioned example of a high-speed search, it is indispensable to accurately count the FG pulses and PG pulse at speeds ranging from 2,400 min.sup.-1 ! to 15,500 min.sup.-1 !.
Since the FG detection signals are signals produced in regular cycles, they are easily detected over a broad speed range by making a comparison with a zero crossing point. On the other hand, since the PG detection signal is a signal which generates only one pulse per rotation of the rotor and its peak value varies in proportion to the rotational speed of the drum, it is difficult to detect the PG pulse over a broad speed range by a conventional PG pulse detecting method.
The following description will explain the conventional PG pulse detecting method.
FIG. 6 is a block diagram showing the circuit structure of a conventional drum phase detector (PG). In the PG of this structure, a reproduced signal detected by a PG detecting coil 111 is amplified to a level permitting signal processing by a differential amplifier 112. The amplified output (PG detection signal) of the differential amplifier 112 is input to the non-inverted input of a comparator 115, and compared with a fixed threshold which is input to the inverted input of the comparator 115 from a terminal 114. A digital signal (PG pulse) which shows a logic state "1" when the PG detection signal is greater than the fixed threshold, and shows a logic state "0" when the PG detection signal is smaller than the fixed threshold, is output to a terminal 116 from the comparator 115.
A specific problem of the PG pulse detecting method using the PG shown in FIG. 6 will be explained below.
In the rotating drum, driving magnets are magnetized in a radial direction so that N poles and S poles which account for eight poles in total are alternately arranged. In addition, four PG forming poles as PG forming magnets are magnetized in a direction perpendicular to the magnetized direction of the respective driving magnets. The PG detecting coil 111 is a pattern having a shape of a substantially square bracket, and mounted on a position of a stator (fixed section) of the drum motor so that the PG detecting coil 111 faces the PG forming magnets when the PG forming magnets rotate and pass the PG detecting coil 111.
In the above-mentioned structure, in the vicinity of the time point at which the PG forming magnets and the PG detecting coil 111 face each other, the relative speed of the PG forming magnets to the PG detecting coil shows the greatest change. Therefore, as shown in FIGS. 4(a) to 4(c), at this time point, a main pulse 65 appears in the waveform of the PG detection signal. FIGS. 4(a) to 4(c) are the results of simulations, showing the waveform of the PG detection signal when the rotor was rotated in a counterclockwise direction in the rotating drum of the above-mentioned structure. The horizontal axis in each figure shows time, and the vertical axis shows values which are normalized by using 1 as a maximum value of the peak of the PG signal waveform when the number of rotations of the drum is 9,000 min.sup.-1 !. FIG. 4(a) shows the PG detection signal waveform when the number of rotations of the drum is 9,000 min.sup.-1 !. Similarly, FIG. 4(b) shows the PG detection signal waveform when the number of rotations of the drum is 15,500 min.sup.-1 !. FIG. 4(c) shows the PG detection signal waveform when the number of rotations of the drum is 2,400 min.sup.-1 !.
It is clear from FIGS. 4(a) to 4(c) that the PG detection signal waveform repeats overshoot and undershoot, and has not only the main pulse 65, but also spurious pulses on both sides of the main pulse 65. The numeric value 61 shown in FIGS. 4(a) to 4(c) represents a fixed threshold level from the terminal 114 shown in FIG. 6.
In the PG detection signal obtained when the number of rotations of the drum is 9,000 min.sup.-1 !, as shown in FIG. 4(a), since only the main pulse 65 exceeds the threshold level 61, it is possible to obtain a correct PG pulse only in the portion of the main pulse 65 by the comparator 115. However, if the number of rotations of the drum is increased to 15,500 min.sup.-1 !, as shown in FIG. 4(b), since the peak value of the spurious pulses 64 on both sides of the main pulse 65 of the PG detection signal increases and exceeds the threshold level 61, PG pulses are generated at unnecessary positions. When the rotating drum is rotated at a low speed (for example, the number of rotations of the drum is 2400 min.sup.-1 !), as shown in FIG. 4(c), the output of the PG detection signal is lowered, and even the main pulse 65 cannot exceed the threshold level 61. Thus, the PG pulse cannot be generated.
Namely, the drum phase detector of the conventional magnetic recording and reproduction apparatus has difficulty in accurately generating the PG pulse over a broad range of the drum rotational speed.
As a method for eliminating the spurious pulses generated when the drum is rotated at high speeds, a method (Japanese Publication for Examined Patent Application No. 21843/1995 (Tokukohei 7-21843)) used as a signal processing method in a magnetic recording device using a thin-film head may be adopted. This method eliminates the spurious pulses by circulation by using a structure formed by a plurality of delay circuits for introducing delays in the transmission of the detection signal, a plurality of amplitude regulating circuits for regulating the amplitudes of the outputs from the delay circuits, and an adder for providing the sum of the outputs from the amplitude regulating circuits. This structure is exactly the same as a so-called transversal filter. In the delay circuits, the delay time is determined so that the peak value of the main pulse and that of the spurious pulse overlap. It is possible to eliminate the spurious pulse in an arbitrary position by adjusting the delay time to an optimum value.
An example of the structure of the PG using the above-mentioned delay circuits is illustrated in FIG. 7. In this structure, the output of the PG detecting coil 111 is amplified by the differential amplifier 112, and then input to a delay circuit 121 and an amplitude regulating circuit 123. The output of the delay circuit 121 is input to a delay circuit 122. Further, the output of the delay circuit 122 is input to an amplitude regulating circuit 124. Then, the outputs of the amplitude regulating circuits 123 and 124 and the output of the delay circuit 121 are respectively input to a waveform synthesizing circuit 125. The waveform synthesizing circuit 125 outputs a signal produced by eliminating the spurious pulses based on the input signal. The output of the waveform synthesizing circuit 125 is input to the non-inverted input of the comparator 115, and the level of the output is compared with the fixed threshold input to the inverted input from the terminal 114. As a result, the PG pulse is output from the terminal 116. It is necessary to provide the same number of delay circuits as the number of spurious pulses to be eliminated.
However, in a magnetic recording and reproducing apparatus using the technique disclosed in the above-mentioned publication (Tokukohei 7-21843), since the delay time in the delay circuits is uniform, the spurious peak cannot be certainly eliminated in a high-speed search which is performed by broadly changing the number of rotations of the drum according to the feed speed and feed direction of the magnetic tape.
This problem can be solved by changing the structure so that the delay time in the delay circuits becomes variable. However, if there are a number of spurious peaks that exceed the threshold, a plurality of delay circuits for introducing variable delays are required. Thus, this structure causes a new problem, namely, an increase in the cost.